'\" et
.TH READ "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
pread,
read
\(em read from a file
.SH SYNOPSIS
.LP
.nf
#include <unistd.h>
.P
ssize_t pread(int \fIfildes\fP, void *\fIbuf\fP, size_t \fInbyte\fP, off_t \fIoffset\fR);
ssize_t read(int \fIfildes\fP, void *\fIbuf\fP, size_t \fInbyte\fP);
.fi
.SH DESCRIPTION
The
\fIread\fR()
function shall attempt to read
.IR nbyte
bytes from the file associated with the open file descriptor,
.IR fildes ,
into the buffer pointed to by
.IR buf .
The behavior of multiple concurrent reads on the same pipe, FIFO, or
terminal device is unspecified.
.P
Before any action described below is taken, and if
.IR nbyte
is zero, the
\fIread\fR()
function may detect and return errors as described below. In the
absence of errors, or if error detection is not performed, the
\fIread\fR()
function shall return zero and have no other results.
.P
On files that support seeking (for example, a regular file), the
\fIread\fR()
shall start at a position in the file given by the file offset
associated with
.IR fildes .
The file offset shall be incremented by the number of bytes
actually read.
.P
Files that do not support seeking\(emfor example, terminals\(emalways
read from the current position. The value of a file offset associated
with such a file is undefined.
.P
No data transfer shall occur past the current end-of-file. If the
starting position is at or after the end-of-file, 0 shall be returned.
If the file refers to a device special file, the result of subsequent
\fIread\fR()
requests is implementation-defined.
.P
If the value of
.IR nbyte
is greater than
{SSIZE_MAX},
the result is implementation-defined.
.P
When attempting to read from an empty pipe or FIFO:
.IP " *" 4
If no process has the pipe open for writing,
\fIread\fR()
shall return 0 to indicate end-of-file.
.IP " *" 4
If some process has the pipe open for writing and O_NONBLOCK is set,
\fIread\fR()
shall return \-1 and set
.IR errno
to
.BR [EAGAIN] .
.IP " *" 4
If some process has the pipe open for writing and O_NONBLOCK is clear,
\fIread\fR()
shall block the calling thread until some data is written or the pipe
is closed by all processes that had the pipe open for writing.
.P
When attempting to read a file (other than a pipe or FIFO) that
supports non-blocking reads and has no data currently available:
.IP " *" 4
If O_NONBLOCK is set,
\fIread\fR()
shall return \-1 and set
.IR errno
to
.BR [EAGAIN] .
.IP " *" 4
If O_NONBLOCK is clear,
\fIread\fR()
shall block the calling thread until some data becomes available.
.IP " *" 4
The use of the O_NONBLOCK flag has no effect if there is some data
available.
.P
The
\fIread\fR()
function reads data previously written to a file. If any portion of a
regular file prior to the end-of-file has not been written,
\fIread\fR()
shall return bytes with value 0. For example,
\fIlseek\fR()
allows the file offset to be set beyond the end of existing data in the
file. If data is later written at this point, subsequent reads in the
gap between the previous end of data and the newly written data shall
return bytes with value 0 until data is written into the gap.
.P
Upon successful completion, where
.IR nbyte
is greater than 0,
\fIread\fR()
shall mark for update the last data access timestamp
of the file, and shall return the number of bytes read.
This number shall never be greater than
.IR nbyte .
The value returned may be less than
.IR nbyte
if the number of bytes left in the file is less than
.IR nbyte ,
if the
\fIread\fR()
request was interrupted by a signal, or if the file is a pipe or FIFO
or special file and has fewer than
.IR nbyte
bytes immediately available for reading. For example, a
\fIread\fR()
from a file associated with a terminal may return one typed line of
data.
.P
If a
\fIread\fR()
is interrupted by a signal before it reads any data, it shall return
\-1 with
.IR errno
set to
.BR [EINTR] .
.P
If a
\fIread\fR()
is interrupted by a signal after it has successfully read some data, it
shall return the number of bytes read.
.P
For regular files, no data transfer shall occur past the offset maximum
established in the open file description associated with
.IR fildes .
.P
If
.IR fildes
refers to a socket,
\fIread\fR()
shall be equivalent to
\fIrecv\fR()
with no flags set.
.P
If the O_DSYNC and O_RSYNC bits have been set,
read I/O operations on the file descriptor shall complete as defined by
synchronized I/O data integrity completion. If the O_SYNC and O_RSYNC
bits have been set, read I/O operations on the file descriptor shall
complete as defined by synchronized I/O file integrity completion.
.P
If
.IR fildes
refers to a shared memory object, the result of the
\fIread\fR()
function is unspecified.
.P
If
.IR fildes
refers to a typed memory object, the result of the
\fIread\fR()
function is unspecified.
.P
A
\fIread\fR()
from a STREAMS file can read data in three different modes:
\fIbyte-stream\fP mode, \fImessage-nondiscard\fP mode, and
\fImessage-discard\fP mode. The default shall be byte-stream mode.
This can be changed using the I_SRDOPT
\fIioctl\fR()
request, and can be tested with I_GRDOPT
\fIioctl\fR().
In byte-stream mode,
\fIread\fR()
shall retrieve data from the STREAM until as many bytes as were
requested are
transferred, or until there is no more data to be retrieved.
Byte-stream mode ignores message boundaries.
.P
In STREAMS message-nondiscard mode,
\fIread\fR()
shall retrieve data until as many bytes as were requested are
transferred, or until a message boundary is reached. If
\fIread\fR()
does not retrieve all the data in a message, the remaining data shall
be left on the STREAM, and can be retrieved by the next
\fIread\fR()
call. Message-discard mode also retrieves data until as many bytes as
were requested are transferred, or a message boundary is reached.
However, unread data remaining in a message after the
\fIread\fR()
returns shall be discarded, and shall not be available for a subsequent
\fIread\fR(),
\fIgetmsg\fR(),
or
\fIgetpmsg\fR()
call.
.P
How
\fIread\fR()
handles zero-byte STREAMS messages is determined by the current read
mode setting. In byte-stream mode,
\fIread\fR()
shall accept data until it has read
.IR nbyte
bytes, or until there is no more data to read, or until a zero-byte
message block is encountered. The
\fIread\fR()
function shall then return the number of bytes read, and place the
zero-byte message back on the STREAM to be retrieved by the next
\fIread\fR(),
\fIgetmsg\fR(),
or
\fIgetpmsg\fR().
In message-nondiscard mode or message-discard mode, a zero-byte message
shall return 0 and the message shall be removed from the STREAM. When a
zero-byte message is read as the first message on a STREAM, the message
shall be removed from the STREAM and 0 shall be returned, regardless of
the read mode.
.P
A
\fIread\fR()
from a STREAMS file shall return the data in the message at the front
of the STREAM head read queue, regardless of the priority band of the
message.
.P
By default, STREAMs are in control-normal mode, in which a
\fIread\fR()
from a STREAMS file can only process messages that contain a data part
but do not contain a control part. The
\fIread\fR()
shall fail if a message containing a control part is encountered at the
STREAM head. This default action can be changed by placing the STREAM
in either control-data mode or control-discard mode with the I_SRDOPT
\fIioctl\fR()
command. In control-data mode,
\fIread\fR()
shall convert any control part to data and pass it to the application
before passing any data part originally present in the same message.
In control-discard mode,
\fIread\fR()
shall discard message control parts but return to the process any data
part in the message.
.P
In addition,
\fIread\fR()
shall fail if the STREAM head had processed an asynchronous error
before the call. In this case, the value of
.IR errno
shall not reflect the result of
\fIread\fR(),
but reflect the prior error. If a hangup occurs on the STREAM being
read,
\fIread\fR()
shall continue to operate normally until the STREAM head read queue is
empty. Thereafter, it shall return 0.
.P
The
\fIpread\fR()
function shall be equivalent to
\fIread\fR(),
except that it shall read from a given position in the file without
changing the file offset. The first three arguments to
\fIpread\fR()
are the same as
\fIread\fR()
with the addition of a fourth argument
.IR offset
for the desired position inside the file. An attempt to perform a
\fIpread\fR()
on a file that is incapable of seeking shall result in an error.
.SH "RETURN VALUE"
Upon successful completion, these functions shall return a non-negative
integer indicating the number of bytes actually read. Otherwise, the
functions shall return \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
These functions shall fail if:
.TP
.BR EAGAIN
The file is neither a pipe, nor a FIFO, nor a socket, the O_NONBLOCK
flag is set for the file descriptor, and the thread would be delayed
in the read operation.
.TP
.BR EBADF
The
.IR fildes
argument is not a valid file descriptor open for reading.
.TP
.BR EBADMSG
The file is a STREAM file that is set to control-normal mode and the
message waiting to be read includes a control part.
.TP
.BR EINTR
The read operation was terminated due to the receipt of a signal, and
no data was transferred.
.TP
.BR EINVAL
The STREAM or multiplexer referenced by
.IR fildes
is linked (directly or indirectly) downstream from a multiplexer.
.TP
.BR EIO
The process is a member of a background process group attempting to read
from its controlling terminal, and either the calling thread is blocking
SIGTTIN or the process is ignoring SIGTTIN or the process group of the
process is orphaned. This error may also be generated for
implementation-defined reasons.
.TP
.BR EISDIR
The
.IR fildes
argument refers to a directory and the implementation
does not allow the directory to be read using
\fIread\fR()
or
\fIpread\fR().
The
\fIreaddir\fR()
function should be used instead.
.TP
.BR EOVERFLOW
The file is a regular file,
.IR nbyte
is greater than 0, the starting position is before the end-of-file, and
the starting position is greater than or equal to the offset maximum
established in the open file description associated with
.IR fildes .
.P
The
\fIpread\fR()
function shall fail if:
.TP
.BR EINVAL
The file is a regular file or block special file, and the
.IR offset
argument is negative. The file offset shall remain unchanged.
.TP
.BR ESPIPE
The file is incapable of seeking.
.br
.P
The
\fIread\fR()
function shall fail if:
.TP
.BR EAGAIN
The file is a pipe or FIFO, the O_NONBLOCK flag is set for the file
descriptor, and the thread would be delayed in the read operation.
.TP
.BR EAGAIN " or " EWOULDBLOCK
.br
The file is a socket, the O_NONBLOCK flag is set for the file
descriptor, and the thread would be delayed in the read operation.
.TP
.BR ECONNRESET
A read was attempted on a socket and the connection was forcibly closed
by its peer.
.TP
.BR ENOTCONN
A read was attempted on a socket that is not connected.
.TP
.BR ETIMEDOUT
A read was attempted on a socket and a transmission timeout occurred.
.P
These functions may fail if:
.TP
.BR EIO
A physical I/O error has occurred.
.TP
.BR ENOBUFS
Insufficient resources were available in the system to perform the
operation.
.TP
.BR ENOMEM
Insufficient memory was available to fulfill the request.
.TP
.BR ENXIO
A request was made of a nonexistent device, or the request was outside
the capabilities of the device.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Reading Data into a Buffer"
.P
The following example reads data from the file associated with the file
descriptor
.IR fd
into the buffer pointed to by
.IR buf .
.sp
.RS 4
.nf

#include <sys/types.h>
#include <unistd.h>
\&...
char buf[20];
size_t nbytes;
ssize_t bytes_read;
int fd;
\&...
nbytes = sizeof(buf);
bytes_read = read(fd, buf, nbytes);
\&...
.fi
.P
.RE
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
This volume of POSIX.1\(hy2017 does not specify the value of the file offset after an
error is returned; there are too many cases. For programming errors,
such as
.BR [EBADF] ,
the concept is meaningless since no file is involved. For errors that
are detected immediately, such as
.BR [EAGAIN] ,
clearly the offset should not change. After an interrupt or hardware
error, however, an updated value would be very useful and is the
behavior of many implementations.
.P
Note that a
\fIread\fR()
of zero bytes does not modify the last data access timestamp. A
\fIread\fR()
that requests more than zero bytes, but returns zero, is required
to modify the last data access timestamp.
.P
Implementations are allowed, but not required, to perform error
checking for
\fIread\fR()
requests of zero bytes.
.SS "Input and Output"
.P
The use of I/O with large byte counts has always presented problems.
Ideas such as
\fIlread\fR()
and
\fIlwrite\fR()
(using and returning
.BR long s)
were considered at one time. The current solution is to use abstract
types on the ISO\ C standard function to
\fIread\fR()
and
\fIwrite\fR().
The abstract types can be declared so that existing functions work, but
can also be declared so that larger types can be represented in future
implementations. It is presumed that whatever constraints limit the
maximum range of
.BR size_t
also limit portable I/O requests to the same range. This volume of POSIX.1\(hy2017 also limits
the range further by requiring that the byte count be limited so that a
signed return value remains meaningful. Since the return type is also a
(signed) abstract type, the byte count can be defined by the
implementation to be larger than an
.BR int
can hold.
.P
The standard developers considered adding atomicity requirements to a
pipe or FIFO, but recognized that due to the nature of pipes and FIFOs
there could be no guarantee of atomicity of reads of
{PIPE_BUF}
or any other size that would be an aid to applications portability.
.P
This volume of POSIX.1\(hy2017 requires that no action be taken for
\fIread\fR()
or
\fIwrite\fR()
when
.IR nbyte
is zero. This is not intended to take precedence over detection of
errors (such as invalid buffer pointers or file descriptors). This is
consistent with the rest of this volume of POSIX.1\(hy2017, but the phrasing here could be
misread to require detection of the zero case before any other errors.
A value of zero is to be considered a correct value, for which the
semantics are a no-op.
.P
I/O is intended to be atomic to ordinary files and pipes and FIFOs.
Atomic means that all the bytes from a single operation that
started out together end up together, without interleaving from other
I/O operations. It is a known attribute of terminals that this is not
honored, and terminals are explicitly (and implicitly permanently)
excepted, making the behavior unspecified. The behavior for other
device types is also left unspecified, but the wording is intended to
imply that future standards might choose to specify atomicity (or not).
.P
There were recommendations to add format parameters to
\fIread\fR()
and
\fIwrite\fR()
in order to handle networked transfers among heterogeneous file system
and base hardware types. Such a facility may be required for support by
the OSI presentation of layer services. However, it was determined that
this should correspond with similar C-language facilities, and that is
beyond the scope of this volume of POSIX.1\(hy2017. The concept was suggested to the developers
of the ISO\ C standard for their consideration as a possible area for future
work.
.P
In 4.3 BSD, a
\fIread\fR()
or
\fIwrite\fR()
that is interrupted by a signal before transferring any data does not
by default return an
.BR [EINTR] 
error, but is restarted. In 4.2 BSD,
4.3 BSD,
and the Eighth Edition, there is an additional function,
\fIselect\fR(),
whose purpose is to pause until specified activity (data to read, space
to write, and so on) is detected on specified file descriptors. It is
common in applications written for those systems for
\fIselect\fR()
to be used before
\fIread\fR()
in situations (such as keyboard input) where interruption of I/O due to
a signal is desired.
.P
The issue of which files or file types are interruptible is considered
an implementation design issue. This is often affected primarily by
hardware and reliability issues.
.P
There are no references to actions taken following an ``unrecoverable
error''. It is considered beyond the scope of this volume of POSIX.1\(hy2017 to describe what
happens in the case of hardware errors.
.P
Earlier versions of this standard allowed two very different behaviors
with regard to the handling of interrupts. In order to minimize the
resulting confusion, it was decided that POSIX.1\(hy2008 should support only one
of these behaviors. Historical practice on AT&T-derived systems was to
have
\fIread\fR()
and
\fIwrite\fR()
return \-1 and set
.IR errno
to
.BR [EINTR] 
when interrupted after some, but not all, of the data requested had
been transferred. However, the US Department of Commerce FIPS 151\(hy1 and
FIPS 151\(hy2 require the historical BSD behavior, in which
\fIread\fR()
and
\fIwrite\fR()
return the number of bytes actually transferred before the interrupt.
If \-1 is returned when any data is transferred, it is difficult to
recover from the error on a seekable device and impossible on a
non-seekable device. Most new implementations support this behavior.
The behavior required by POSIX.1\(hy2008 is to return the number of bytes
transferred.
.P
POSIX.1\(hy2008 does not specify when an implementation that buffers
\fIread\fR()s
actually moves the data into the user-supplied buffer, so an
implementation may choose to do this at the latest possible moment.
Therefore, an interrupt arriving earlier may not cause
\fIread\fR()
to return a partial byte count, but rather to return \-1 and set
.IR errno
to
.BR [EINTR] .
.P
Consideration was also given to combining the two previous options, and
setting
.IR errno
to
.BR [EINTR] 
while returning a short count. However, not only is there no existing
practice that implements this, it is also contradictory to the idea
that when
.IR errno
is set, the function responsible shall return \-1.
.P
This volume of POSIX.1\(hy2017 intentionally does not specify any
\fIpread\fR()
errors related to pipes, FIFOs, and sockets other than
.BR [ESPIPE] .
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIfcntl\fR\^(\|)",
.IR "\fIioctl\fR\^(\|)",
.IR "\fIlseek\fR\^(\|)",
.IR "\fIopen\fR\^(\|)",
.IR "\fIpipe\fR\^(\|)",
.IR "\fIreadv\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Chapter 11" ", " "General Terminal Interface",
.IR "\fB<stropts.h>\fP",
.IR "\fB<sys_uio.h>\fP",
.IR "\fB<unistd.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
